Deep in Mumy's uterus I am growing at a furious pace. At this point I'm about the size of a sesame seed, and I look more like a tiny tadpole than a human. I'm now made up of three layers — the ectoderm, mesoderm, and endoderm — that will later form my organs and tissues. The cells are forming for my major organs, including my kidneys and liver, and my neural tube is beginning to develop. This neural tube — from which your my brain, spinal cord, nerves, and backbone will sprout — develops in the top layer, called the ectoderm. This layer will also give rise to my skin, hair, nails, mammary and sweat glands, and tooth enamel. My heart and circulatory system begin to form in the middle layer, or mesoderm. (This week, in fact, my tiny heart begins to divide into chambers and beat and pump blood.) The mesoderm will also form your my muscles, cartilage, bone, and subcutaneous (under skin) tissue. The third layer, or endoderm, will house my lungs, intestines, and rudimentary urinary system, as well as my thyroid, liver, and pancreas. In the meantime, the primitive placenta and umbilical cord, which deliver nourishment and oxygen to me, are already on the job.
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